1. Field of the Invention
The present invention relates to semiconductor wafer process chambers and, more particularly, to a heat shield for shielding a device such as a pump from thermal energy generated within a semiconductor wafer process chamber.
2. Background of the Invention
To produce a sufficient vacuum for processing semiconductor wafers in a process chamber of a semiconductor wafer processing system, a first and second stage pump down is performed. Generally, in the first stage, the chamber is evacuated and brought to a first vacuum level. After the bulk of the atmosphere has been removed from the chamber and a vacuum is established, the second stage is initiated. During the second stage, a cryogenic pump (commonly referred to as a cryo pump) is used to attain a high vacuum level within the process chamber. Systems that utilize cryo pumps to achieve high vacuums include physical vapor deposition (PVD) systems that require base pressures (i.e., without back filling will sputtering gases) on the order of 10xe2x88x929 Torr, to obtain optimal process conditions and process performance.
Generally, the cryo pump develops a high vacuum within the chamber by removing molecules and other gases remaining in the chamber atmosphere after the first stage pump down. The cryo pump typically comprises a plurality of vane arrays. Each vane in each array is fabricated from a material, that when at a low temperature, adsorbs molecules and other gases that come in contact with the vane during the pumping process. It should be noted that only a finite number of molecules can be adsorbed by the cryo pump, making the capacity pump sensitive to loading from sources other than chamber atmosphere. At a point in processing, preferably after a relatively large number of wafers have been processed in the vacuum environment, the cryo pump is heated to discharge, i.e., desorb or off-gas, the collected molecules and other gases adsorbed during pumping. Generally stated, the cryo pump adsorbs gases when cold, then progressively loses its ability to adsorb gases as the cryo pump temperature increases, until reaching a temperature where the cryo pump desorbs gases. As such, the temperature of the pump directly effects the ability of the cryo pump to achieve and maintain high vacuum (i.e., the cryo pump must remain cool to efficiently achieve high vacuum).
Typically, the cryo pump is connected to a port in the process chamber via an elbow conduit. The elbow conduit functions to protect the cryo pump from heat generated in the chamber by lamps, pedestal heaters, plasma and other heat sources within the chamber. The elbow conduit thermally xe2x80x9cisolatesxe2x80x9d the cryo pump by placing the cryo pump at a distance from the chamber where the heating effects from the chamber are less severe. Additionally, the xe2x80x9celbowxe2x80x9d shape of the conduit positions the cryo pump out of direct incidence of radiant energy exiting the chamber through the port. The port is also typically fitted with a shield to reflect radiant energy generated within the process chamber.
Before a process chamber is used to process semiconductor wafers, the chamber goes through a process known as xe2x80x9cbakeoutxe2x80x9d, where the chamber is heated by lamps to desorb and evaporate any volatile particles from the surfaces exposed to the interior of the chamber. The removal of these particles is important to both the ability to achieve a high vacuum and to minimize contamination of substrates processed within the chamber.
Once the volatile particles have been pumped from the chamber, the chamber is allowed to cool to a nominal temperature over a period of time known as the cooldown period. A chamber is considered xe2x80x9cqualifiedxe2x80x9d for processing wafers when the chamber achieves and can maintain a vacuum on the order of 8xc3x9710xe2x88x929 to 5xc3x9710xe2x88x929 Torr after both bakeout and cooldown cycles are complete.
A number of problems have been identified in systems utilizing cryo pumps that contribute to difficulty in achieving and maintaining high vacuums. One problem is the difficulty in desorbing volatiles and other contaminants from the elbow conduit. The position of the elbow conduit intermediate of the chamber and cryo pump impedes the heating of the elbow""s surfaces required to remove contaminants from the elbow conduit during bakeout. As a result, the elbow conduit may outgas material that loads the cryo pump before a high vacuum is achieved, i.e., it may continue to outgas at a relatively high rate when outgassing of the chambers surfaces reaches a relatively low rate. Furthermore, the curved geometry of the elbow conduit causes molecules and other contaminates to impinge on the elbow conduit""s interior when exiting the chamber through the port. These molecules and contaminants later become dislodged and cause the cryo pump not to be able to reach the desire vacuum level or cause the vacuum pressure to drift due to molecular loading of the cryo pump from outgassed elbow materials.
Another problem is that the heat shield is typically fabricated from aluminum. The aluminum heat shield heats rapidly, and eventually becomes a heat source relative to the cryo pump due to the shield""s proximity to the pump. Fluid channels within the aluminum shield, or the shield mounting flange for cooling have been disfavored since fluid channels located within the vacuum environment may result in catastrophic chamber contamination if fluid were to leak.
These problems aggregate to cause long qualifying times during chamber bakeout and inhibit the ability of the cryo pump to reach and maintain high vacuum levels. This results in reduced tool capacity, and consequently less product throughput and increased production costs.
Therefore, there is a need in the art for a heat shield that enhances the performance of a cryo pump by shielding the pump from thermal energy generated within a semiconductor wafer process chamber.
The disadvantages heretofore associated with the prior art are overcome by the present invention of apparatus, positioned at an inlet port of a device such as a cryo pump, for shielding the cryo pump from a process chamber of a semiconductor wafer processing system. Specifically, the apparatus comprises a shield member coupled to a mounting flange. The mounting flange comprises a fluid passage positioned outside of a sealing area such that a seal failure will not result in contamination of the process chamber. The fluid passage is adapted for flowing a fluid to transfer heat to/from the shield member.
In one aspect of the invention, the inventive apparatus reflects thermal energy generated within the process chamber while transferring thermal energy absorbed by the heat shield to a heat transfer fluid. Thus the inventive shield allows the cryo pump to be mounted close to the chamber while allowing for high vacuum to be maintained in the chamber through increased efficiency of the cryo pump.